1. Technical Field
The present disclosure relates to a variable geometry system turbocharger equipped with a variable nozzle unit which adjusts a passage area (a throat area) for exhaust gas to be supplied to a turbine wheel side.
2. Description of the Related Art
A variable geometry system turbocharger is equipped with a variable nozzle unit which adjusts a passage area (a throat area) for exhaust gas to be supplied to a turbine wheel side (see Japanese Patent Application Laid-open Publication Nos. 2013-130116 and 2013-194546). The variable nozzle unit is disposed between a turbine scroll passage and the turbine wheel in a turbine housing adjacent to a bearing housing. Configurations of the conventional variable nozzle unit and its surrounding components are as follows.
A first nozzle ring is disposed in the turbine housing. A second nozzle ring is provided integrally with the first nozzle ring at a position away from the first nozzle ring in an axial direction (an axial direction of the turbine wheel). The first nozzle ring has a surface (an opposed surface) which is opposed to the second nozzle ring. Likewise, the second nozzle ring has a surface (an opposed surface) which is opposed to the first nozzle ring. Moreover, variable nozzles are disposed between the opposed surface of the first nozzle ring and the opposed surface of the second nozzle ring. The variable nozzles are disposed at intervals in a circumferential direction (a predetermined circumferential direction). Each variable nozzle is rotatable in forward and reverse directions (opening and closing directions) around a shaft center which is parallel to a shaft center of the turbine wheel. A link mechanism is disposed on an opposite surface side from the opposed surface of the first nozzle ring. The link mechanism rotates the variable nozzles synchronously in the forward and reverse directions. The passage area (the throat area) of the exhaust gas to be supplied to the turbine wheel side is increased when the link mechanism rotates the variable nozzles synchronously in the forward direction (the opening direction). On the other hand, the passage area is reduced when the link mechanism rotates the variable nozzles synchronously in the reverse direction (the closing direction).
The bearing housing has a side surface which is opposed to a back surface of the turbine wheel. An annular protrusion is formed at a central part of the side surface. The protrusion protrudes toward the back surface of the turbine wheel. Meanwhile, an annular heat shield plate is fitted to an outer peripheral surface of the protrusion of the bearing housing. The heat shield plate shields heat from the back surface side of the turbine wheel. A biasing member is provided at a position on the outer peripheral surface of the protrusion of the bearing housing, the position being adjacent to the heat shield plate. The biasing member is formed from a wave washer or the like, which biases the heat shield plate in a direction to bring the heat shield plate into pressure contact with an inner peripheral edge portion of the first nozzle ring. Here, a press-contacting portion between the first nozzle ring and the heat shield plate is a sealing portion for suppressing a leakage of the exhaust gas from the opposite surface side from the opposed surface of the first nozzle ring to an inlet side of the turbine wheel.